I only have an android phone so report only on the android Gaia GPS and Backcountry Navigator versions.
February 2019: Gaia GPS vs Backcountry Navigator
FWIW, in 2019 I installed both
Gaia GPS (basic subscription)
and
Backcountry Navigator (Pro)
and tested both in Big Sur (using off-line stored maps) and elsewhere (using on-line maps).
For my Samsung android 8 (oreo), Gaia kept producing very spikey, essentially unusable, tracks (example)
whereas Backcountry Navigator worked well.
(As of May 2019, Gaia support says they are aware of the problem but do not have fix.)
And in general I found BCN more usable (and had fewer problems), so have ended up using that.
I've also heard others tell of unfixable Gaia problems, but those are hearsay so will not repeat them here.
March 2020: Gaia GPS Update and Warning
My updated experience with Gaia GPS app, to give it another try as a paid subscriber, has been spotty.
It wants to do much automatically - but should something go wrong you can
get zombie processes which cannot be killed.
At the end I could not delete any of their maps I had downloaded and Gaia
became unworkable - and their support could not fix the problem, I had to figure out a solution myself.
Part of the problem may have been with Google doing its own backup syncing - which also
automatically restores data if an app is re-installed.
With all this syncing back-and-forth by different companies, something can
get out of whack and be difficult to fix should that happen.
Beyond technical problems, be aware that Gaia is uploading all
your data for them to analyze, including tracking data. You aren't told about this when
installing Gaia, but it creates an on-line account page and will try to keep syncing it with all the maps, etc. on your phone - and if something goes wrong with that syncing
you are in trouble. That will be occurring by default.
You can turn off this syncing and tracking. Since there are several
different syncing/tracking features there are different buttons which control each.
To turn off all map syncing you must go the phone's "Account" screen (not an
intuitive place to find it). Tracking buttons are found at "Settings - Other".
There are also similar buttons on the website.
Personally I think Gaia is a bit technically and ethically challenged, so
am staying away and will use Avenza and Back Country Navigator - less fancy, but they just work.
But having damned Gaia GPS, let me say something favorable.
From a technical view, Gaia is pioneering the use of vector maps. Besides giving smaller map files with quicker
downloads, potentially the use of digital date will allow the app to have features previously limited
to a GPS (though still unimplemented). I give them high marks for
these efforts. [Backcountry Navigator currently is pursuing the same approach in their "XE" version, but its
software development is behind Gaia's and is buggy.]
And Gaia's
"cloud" allows the user to have data on all "devices", which can be very useful.
(But such syncing is tricky - and with Gaia the user can be the guinea pig who finds the "bugs".)
Gaia is also producing their own topo map which is, IMHO, for hiking more
readable and usable than the newer USGS maps and other on-line-type background maps.
And finally, you get good support from Gaia (though sometimes they themselves have difficulty figuring what is happening).
Bottom line: Gaia can be great when it works, not great when it doesn't.
